1. Field of the Invention
The present invention relates to asphalt surfacing compositions and more particularly to compositions for use in paving construction, which compositions have improved adhesion of the asphalt binder to the mineral aggregate. The improved adhesion is obtained by including in the surfacing composition a silane as described herein below. The silane may be added either to the asphalt or to the mineral aggregate prior to mixing. The compositions show a surprising increase in the adhesion of the binder to the mineral aggregate even when completely immersed in water for extended periods.
2. Description of the Prior Art
A wide variety of silicon compounds including silanes have been used for the impregnation and surface treatment of glass fibers to promote the adhesion of various organic resins such as the polyester resins, epoxide resins or phenol formaldehyde resins and for the surface treatment of textiles, leathers, ceramics and glass materials.
It has been found that the adhesion of asphalt to siliceous aggregates could be markedly improved by first treating the aggregate with vapors of a mixture of methylchlorosilanes. However, this exotic method of application of the silanes to the aggregate is not practical in large scale applications. Sanderson, F. C., "Methylchlorosilanes as Anti-stripping Agents". Proceedings, Highway Research Board, 31, 288 (1952).
U.S. Pat. No. 2,570,185 (106-273) issued Oct. 9, 1951 discloses that the reaction product of aminoalkoxysilanes and high molecular weight aliphatic primary amines containing at least 6 carbon atoms may be added to asphalt to improve the coating properties and anti-stripping properties of the asphalt. The only example of a silane shown in this reference is di-t-butoxy-diamino silane. U.S. Pat. No. 2,985,678 (260-448.8) issued May 23, 1961 discloses that higher alkyl or aryl radicals in silicon compounds progressively lower the stability of the compounds. However, the tertiary butyl radical is shown to increase the stability of the silicon compounds, even in silicon compounds which contain long chain alkyl such as lauryl group.
German Pat. No. 800,685 teaches silanes as asphalt adhesion agents. The useful silanes are said to be of the formula SiR.sub.m X.sub.n wherein X represents a halogen or an alkoxy, R represents an organic residue and m and n represent integers of from 1 to 3. Specific examples of R include methyl, phenyl and 2-chloroethylene.
The patent literature, particularly class 106, subclasses 273 and 277 is replete with attempts to improve the adhesion of asphalt binders to mineral aggregate. The patent literature referring to silicon compound chemistry continually is concerned with the thermal stability of silicon compounds. Thus it may be seen it is highly desirable compounds of any type be found which not only promote the adhesion of asphalt to mineral aggregate but also remain stable over a wide temperatue range and for an extended period. In addition to this it is highly desirable that the adhesion promoters be capable of being used without exotic application methods. Preferably the promoter should be susceptible of introduction into the asphalt in the molten state or application to the mineral aggregate prior to mixing with the asphalt or both.